The present invention relates to a method and system for controlling reheater temperature. In particular, the present invention relates to a method and system for controlling the temperature of steam exiting a reheater tube bundle positioned in the convection pass of a carbonaceous fuel boiler system.
A modern steam generator can include a complex configuration of various thermal and hydraulic units for preheating and evaporating water, and for superheating steam. The surfaces of such units are arranged to facilitate: a complete and efficient fuel combustion while minimizing emissions of particulate and gaseous pollutants; steam generation at a desired pressure, temperature and flow rate; and to maximize recovery of the heat produced upon combustion of the fuel.
Reheaters and superheaters are specially designed tube bundles capable of increasing the temperature of saturated steam. Additionally, reheaters and superheaters are designed to obtain specific steam outlet temperatures, while keeping metal temperatures from becoming too elevated, and limiting steam flow pressure losses.
Essentially, a reheater or a superheater is a single-phase heat exchanger comprising tubes through which steam flows, and across which the combustion or flue gas passes on the outside of the tubes, usually in crossflow. Reheater and superheater tube bundles are often made of a steel alloy capable of withstanding high operating temperatures.
The main difference between a reheater and a superheater can be steam pressure. Thus, in a typical steam drum boiler the reheater steam outlet pressure can be about 580 psi, while for the superheater it can be about 2700 psi.
It can be crucial to heat rate and cycle efficiency of a steam generator, such as a carbonaceous fuel boiler-turbine power plant, to regulate and control reheater steam temperature within narrow limits. For example, a change of only about 35.degree. F. to 40.degree. F. in reheat outlet steam temperature can lead to a change of about 1% in heat rate at 1800 psi pressure.
A number of methods for controlling the temperature of the steam exiting a reheater are known. The known methods all have one or more deficiencies or drawbacks, such as being very expensive and/or resulting in excessive reheater pressure loss.
One method to control reheat steam temperature is to use a gas bypass over the reheater. In this method, two separate flue gas passes are provided in the convection pass of the boiler, one for the superheater and one for the reheater. Dampers are located downstream of each flue gas pass to vary the amount of gas flow over each section. The reheater steam temperature is then controlled by varying the amount of flue gas flow through each of the two convection pass sections. This method has the disadvantage that the dampers are located in a higher temperature (500.degree. F. to 700.degree. F.) dust laden flue gas path making them susceptible to erosion and mechanical failure.
Another method of reheater outlet steam temperature control is by use of external heat exchangers. In this method, a portion of the recirculated solids from a fluidized bed is diverted to an externally mounted fluidized bed heat exchanger in which a section of or a complete reheater is located. By varying the amount of solids flow to the externally mounted heat exchanger, the quantity of heat transfer to the reheater and hence the reheater steam temperature can be controlled. Unfortunately, this method can include a high maintenance solids control valve and the reheat tube surface with the externally mounted heat exchanger can require high steam velocities to ensure that metal temperature limits are not exceeded. This can increase the pressure loss across the heat exchanger.
A further method to control reheat steam temperature is by water spray, also called direct contact attemperation, by which a cooling water spray is added to the fluid entering the reheater. This method can have a negative effect on cycle efficiency. Another method is to use excess air supplied to the boiler for reheater steam temperature control. This method can have a negative effect on boiler efficiency.
It is also known to control reheat steam temperature by the use of gas recirculation. In this method large quantities of flue gases are recirculated to achieve the rated reheater outlet steam temperature. A drawback of this method is that it requires use of a gas recirculation fan for handling a hot dust laden gas and requires additional power consumption.
Finally, it is known to control reheat temperature by dividing the reheat steam flow using a steam bypass to divide or alter steam flow into the reheater, as set forth in U.S. Pat. No. 5,038,568. Inherent in this method is interference with reheat steam flow with resulting negative effect on reheat steam pressure loss and cycle efficiency.
What is needed therefore is a method and system for controlling reheater steam temperature which does not have the deficiencies and drawbacks of known methods and systems, and which in particular does not require any adjustment, alteration or interference with reheater steam flow. Additionally, a method and system for controlling reheat steam temperature with minimal reheater pressure loss and without reheater spray attemperation is needed.